Backlight module structure for LED chip holder

ABSTRACT

The present invention provides a backlight module for the light emitting device (LED) chip holder. The backlight module includes a top guide-light board, a bottom guide-light board, a chip holder with an inclined plane thereon, and a light emitting diode chip. The one side of the top guide-light board and the bottom guide-light board has a reflective sheet, and the light emitting diode chip located on the inclined surface of the chip holder, and further the light emitting diode chip embedded in the bottom guide-light. When the light emitting diode chip emitted incident light source, the light source produces the total reflection on the bottom guide-light board to reflect to the reflective mirror. Then, the light source is reflected to the top guide-light board by the reflective sheet, and is emitted out from the top guide-light board to be a light source for the backlight module to be a light source for the backlight module.

FIELD OF THE INVENTION

The present invention generally relates to a light emitting diodestructure, particularly to indicate to a structure for a LED chip holderwith backlight module thereon.

DESCRIPTION OF THE PRIOR ART

Following the improvement and promotion of display manufacturingtechnology, the features of LCD (Liquid crystal display) is small size,and low radiation. Thus, LCD has been replacing the conventional CRT(cathode ray tube) display gradually. In general, the backlight modulesof the conventional LCD display utilizes the cold cathode fluorescentlamp (CCFL) as the light emitting source. However, the cold cathodefluorescent lamp cannot turn-on under the lower temperature. Therefore,the conventional LCD needs to add the heating apparatus to assist theLCD to turn on at lower temperature. Thus, it is to be valued that thelight emitting diode (LED) to be the backlight module for the lightsource recently.

LED is a light emitting source with small size and higher opticalefficiency. The LCD can apply to the indicator and the variety of lightsource with different colors. In recently, RGB (red, green, and blue)LED has been developed, and can be applied for large display screen. Inaddition, LED display screen can operate under the lower powerconsumption and long operating life.

FIG. 1A and FIG 1B show a conventional LED backlight module. The LEDbacklight module 1100 includes a first reflective sheet 1110A, a topguide-light board 1110 that includes a plurality of marks 1114 thereon,a bottom guide-light board 1112, a first reflective mirror 1116, asecond reflective sheet 1118, a third reflective sheet 1120, a flatmetal substrate 1122, a light emitting diode chip 1124, and a secondreflective mirror 1126. The light emitting diode chip 1124 is positionedon the flat metal substrate 1122. The top guide-light board 1110 iscombined with the bottom guide-light board 1112, and a reflective mirror1116 is located on one side of both the top guide-light board 1110 andthe bottom guide-light board 1112. The second reflective sheet 1118 islocated under the top guide-light board 1110 and the bottom guide-lightboard 1112, and the third reflective sheet 1120 is located under thebottom guide-light board 1112. The flat metal substrate 1122 is locatedunder the bottom guide-light board 1112, and is provided to load thelight emitting diode chip 1124 thereon to release the heat which isgenerated from the light emitting diode chip 1124. The part of the lightemitting diode chip 1124 of chip holder 1100 is combined with the secondreflective mirror 1126, such that the partial of the light emittingdiode 1124 is covered by the second reflective mirror 1126. Herein, theepoxy resin 1128 can fill with the space between the reflective mirror1126 and the flat metal substrate 1122, wherein the refractive index ofepoxy resin 1128 as same as refractive index of the bottom guide-lightboard 1112.

The light source 1130 is emitted through the second reflective sheet1118 from the light emitting diode chip 1124 and the reflection isgenerated by the third reflective sheet 1120. Then, the light source1130 is reflected to the top guide-light board 1110 by way of the firstreflective mirror 1116. Next, the light source 1130 is reflected by thefirst reflective sheet 1110A to the plurality of marks 1114, and thetotal reflection optical path would be destroyed by the plurality ofmarks 1114, such that the light source 1130 will emit out via the topguide-light board 1110.

However, the light emitting diode chip 1124 is positioned parallel thechip holder 1122 (flat metal substrate) as shown in FIG. 1B. When thebacklight module 1100 is provided with the light emitting diode chip1124 that operates, the incident angle of the light source isunfavorable to cause the illumination that will not be unity. Further,the light source also cannot spread the uniform illumination over thetop guide-light board 1110, thus, the illumination efficiency would bereduced when the light emitting diode chip 1124 is positioned parallelthe chip holder 1122.

SUMMARY OF THE INVENTION

According to the invention background described above, a conventionalLED backlight module includes many disadvantages. Thus, the presentinvention provides a backlight module with light emitting diode chip toimprove the illumination efficiency and to provide the high brightness.

It is an object of the present invention to disclose a backlight modulewith a light emitting diode chip. The backlight module includes a bendedmetal substrate to be the chip holder. The light emitting diode chip ispositioned on the inclined plane of the bended metal substrate. Thus,the light emitting diode chip emits the light source at a specificangle, and then the total reflection light source would be generatedamong the bottom guide-light board. The total reflection light sourcewould be destroyed by the plurality of marks on the top guide-lightboard, such that the destroyed total reflection light source would beemitted out from the top guide-light board to be the backlight source.

It is another object of the present invention to disclose a bended metalsubstrate to be the chip holder for the backlight module. The lightemitting diode chip is positioned on the inclined plane of the bendedmetal substrate, and the bended metal substrate is positioned in thecave of bottom guide-light board. After the top guide-light board isassembled the bottom guide-light board, the light emitting diode chipcan emit the light source with high illumination efficiency and highbrightness.

It is a further object of the present invention to embed the lightemitting diode chip in side of the bottom guide-light board. The lightemitting diode chip emits the light source to generate the totalreflection light source among the bottom guide-light board.

A further object of the present invention is to embed the light emittingdiode chip in the side of the bottom guide-light board. Then, the topguide-light board is assembled the bottom guide-light board. The lightsource is emitted from the light emitting diode chip, and is generatedthe reflection among the bottom guide-light board. Then, the totalreflection light source is generated by the plurality of reflectivemirrors on the fours side of the bottom guide-light board.

According to the object, the present invention discloses a backlightmodule with a light emitting diode chip. The backlight module includes atop guide-light board, a bottom guide-light board, and a chip holder. Areflective mirror is located on one side of both the top guide-lightboard and bottom guide-light board. A metal substrate is a chip holderto support the light emitting diode chip, wherein the light emittingdiode chip is positioned on the inclined plane of the metal substrate,which is embedded in the bottom guide-light board, and the flat plane ofthe metal substrate is located under the bottom guide-light board. Whenthe light emitting diode chip emits the light source, the light sourcewill reflect via the reflective mirror to the top guide-light board, andlater, the light source will emit out from the top guide-light board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B show a conventional LED backlight module structure;

FIG. 2 shows the bended metal substrate used as the chip holder tosupport the light emitting diode chip according to the presentinvention;

FIG. 3A shows the backlight module with the RGB light emitting diodechip according to the one preferred embodiment of the present invention;

FIG. 3B shows another preferred embodiment of the backlight module withthe light emitting diode chip according to the present invention;

FIG. 4 illustrates the backlight module with the RGB light emittingdiode chip that emits the RGB light source from the two sides of thebacklight module;

FIG. 5A and FIG. 5B illustrate the backlight module with the RGB lightemitting diode chip as the light source according to the presentinvention;

FIG. 6 illustrates the backlight module with the light emitting diodechip according to another preferred embodiment of the present invention;

FIG. 7 illustrates the backlight module that is provided with the RGBlight emitting diode chip to emit the light source from the two sides ofthe backlight module according to another preferred embodiment o thepresent invention;

FIG. 8 illustrates the embodiment of the backlight module with the lightemitting diode LED chip or white light emitting diode chip as the lightsource according to the present invention;

FIG. 9A illustrates the embodiment of the backlight module with thelight emitting diode chip as the light source according to the presentinvention; and

FIG. 9B shows the completely structure for the backlight module with thelight emitting diode chip that according to the embodiment of FIG. 9A ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows the bended metal substrate to be the chip holder to supportthe light emitting diode chip according to the present invention. Themetal substrate 10 includes an angle between an inclined plane 14 andthe flat plane 16. The light emitting diode chip 12 is positioned on theinclined plane 14 of the metal substrate 10. The included angle betweenthe inclined plane 14 and the flat plane 16 is larger than 90 degree andis smaller than 180 degree. The light source has high brightness andhigh illumination efficiency when the light source is reflected to thetop guide-light board (not shown) at a specific included angle. Inaddition, another function of the metal substrate 10 is dissipated theheat when the light emitting diode chip 12 is operated.

FIG. 3A shows the backlight module with the RGB light emitting diodechip according to the preferred embodiment of the present invention. Asecond reflective sheet 30 is located between the top guide-light board22 and the bottom guide-light board 24, and a third reflective sheet 32is located under the bottom guide-light board 24. When the light source40 is emitted from the RGB light emitting diode chip 36, the function ofboth the second reflective sheet 30 and the third reflective sheet 32used to generate the total reflection light source among the bottomguide-light board 24. Then, the hybrid illumination would be generatedby the total reflection effect. Herein, the shape of the reflectivemirror 28 is dependent upon the refractive index of the bottomguide-light board 24. In addition, the shape of the reflective mirror 28is elliptic, or any shape corresponding to the refractive index of thebottom guide-light board 24. Thus, the flat plane 34B of the chip holder34 is located under the bottom guide-light board 24, and the inclinedplane 34A of the chip holder 34 is located on another side of the bottomguide-light board 24. Because of the chip holder 34 includes an inclinedplane 34A, and a space is positioned between the bottom guide-lightboard 24 and the inclined plane 34A of the chip holder 34. Thus, thespace can be full with the material such as the epoxy resin 38 with aspecific refractive index as same as the refractive index of the bottomguide-light board 24. Therefore, the light source 40 is emitted from thelight emitting diode chip 36 that can improve the incident illuminationefficiency via the different refractive indexes.

When the RGB light emitting diode chip 36 emits the light source 40, thelight source 40 would be introduced by the first reflective sheet 30 andthe second reflective sheet 32 to generate the total reflection lightsource among the bottom guide-light board 24. Then, the total reflectionlight source is reflected to the top guide-light board 22 after thelight source 40 is reflected to the reflective mirror 28. When thereflected light source is reflected to the plurality of marks 26, thetotal reflection optical path of the total reflection light source wouldbe destroyed by the plurality of marks 26. Thus, the destroyed totalreflection light source can emit out from the top guide-light board 22to be a light source for the backlight module.

FIG. 3B shows another preferred embodiment of the backlight module withthe light emitting diode chip according to the present invention. FIG.3B illustrates the backlight module as same as FIG. 3A disclosed. Thedifference between the FIG. 3A and FIG. 3B is that the side of thebottom guide-light board 24 is an inclined plane as in FIG. 3B. Theinclined plane of the bottom guide-light board 24 can assemble with theinclined plane 34A of the chip holder 34, such that the light emittingdiode chip 36 can be embedded in the inclined plane of the bottomguide-light board 24. Therefore, there is no space between the bottomguide-light board 24 and the chip holder 34.

When the light emitting diode chip 36 emits the light source 40, thelight source 40 would be introduced by the second reflective sheet 30and the third reflective sheet 32 to generate the total reflection lightsource among the bottom guide-light board 24. Then, the total reflectionlight source is reflected to the top guide-light board 22 via thereflective mirror 28. The total reflection light source would bereflected to the plurality of marks 26 to destroy the total reflectionoptical path of the total reflection light source 40. Thus, thedestroyed total reflection light source 40 can emit out from the topguide-light board 22 to be a light source for the backlight module.

FIG. 4 illustrates the backlight module with the RGB light emittingdiode chip that emits the RGB light source from the two sides of thebacklight module. The backlight module includes a top guide-light board52, at least two bottom guide-light board 54, at least two reflectivemirror 58, at least four reflective sheets 60A, 60B, 60C, and 60D, atleast two chip holders 62, and at least two light emitting diode chips64. The each two light emitting diode chips 64 is located on theinclined plane 62A of the each of two chip holders 62 respectively. Atleast two bottom guide-light boards 54 are positioned under the twosides of the top guide-light board 52.

A reflective mirror 58 is located on one side of both top guide-lightboard 52 and each of two bottom guide-light boards 54. The shape of thereflective mirror 58 is dependent upon the refractive index of thebottom guide-light board 54. In addition, the shape of the reflectivemirror 58 can be the elliptic, or any shape corresponding to therefractive index of the bottom guide-light board 54. The each two chipholders 62 is located under each two bottom guide-light boards 54respectively. A space is positioned between the top guide-light board52, the bottom guide-light board 54, and the inclined plane 62A of thechip holder 62. The space can fill with the epoxy resin with a specificrefractive index as same as the refractive index of two bottomguide-light boards 54. Thus, the illumination efficiency of the lightsource would not be reduced due to the different refractive indexbetween the epoxy resin and the bottom guide-light board 54.

When the RGB light emitting diode chip 64 is emitted the light source66, the light source 64 would be introduced to the total reflection at aspecific angle by the first reflective sheet 60A, the second reflectivesheet 60B, the third reflective sheet 60C, and the forth reflectivesheet 60D among the two bottom guide-light boards 54. Then, the totalreflection light source is reflected to the top guide-light board 52through the two reflective mirrors 58. Then, the total reflection lightsource is reflected to the plurality of marks 56 to destroy the totalreflection optical path of the total reflection light source, such thatthe destroyed total reflection light source can emit out from the topguide-light board 52 to be a light source for the backlight module.

FIG. 5A and FIG. 5B illustrate the backlight module with the RGB lightemitting diode chip as the light source according to the presentinvention. The backlight module includes a top guide-light board 72, andthe top guide-light board 72 includes a plurality of marks 76 and afirst reflective sheet 72A thereon. The backlight module furtherincludes a bottom guide-light board 74 with a cave 78 therein. The shapeof the cave 78 can be rectangular, and the four sides of the cave 78with an inclined angle y. This inclined angle y corresponding to theincluded angle between the inclined plane 80A of the chip holder 80 andthe plane surface 80B of the chip holder 80. When the top guide-lightboard 72 is assembled with the bottom guide-light board 74, the cave 78of the bottom guide-light board 74 can match the inclined plane 80A ofthe chip holder 80.

Also, the backlight module of this embodiment includes the plurality oftop reflective sheets, the plurality of bottom reflective sheets (notshown in FIG. 5A), the plurality of reflective mirrors (not shown), andthe plurality of light emitting diode chips 82. Each plurality of lightemitting diode chips 82 is located on the each of four inclined planes80A of the chip holder 80 respectively. Then, the plurality of topreflective sheets are located above the bottom guide-light board 74, andthe plurality of bottom reflective sheets (not shown in FIG. 5A) arelocated under on the bottom guide-light board 74. Next, the chip holder80 with the plurality of light emitting diode chips 82 are located onthe four inclined plane of the cave 78 within the bottom guide-lightboard 74. Thus, the inclined plane 80A of the chip holder 80 can linkwith the bottom guide-light board 74. Thus, the plurality of lightemitting diode chips 82 would be embedded in the bottom guide-lightboard 74 directly, and there is no space between the chip holder 80 andthe bottom guide-light board 74 as shown in FIG. 5B.

FIG. 6 illustrates the backlight module with the light emitting diodechip according to another preferred embodiment of the present invention.The metal substrate 90 is to be the chip holder, and the light emittingdiode chip 92 is located on an inclined plane of metal substrate 90. Anincluded angle is included between the inclined plane and the flat planeof the chip holder 90. The light emitting diode chip 92 can emit thelight source with high illumination efficiency and high brightness at aspecific included angle. The function of the metal substrate 90 is usedto dissipate the heat that is generated from the light emitting diodechip 92. In addition, the types of the light emitting diode chip 92 isRGB light emitting diode chip or white light emitting diode chip.

FIG. 7 illustrates the backlight module with the RGB light emittingdiode chip to emit the light source from the two sides of the backlightmodule according to another preferred embodiment the present invention.The backlight module includes a top guide-light board 94, a plurality ofmarks 96 that located within the top guide-light board 94, and a firsttop reflective sheet 94 that located on the top guide-light board 94.The backlight module further includes a bottom guide-light board 98, areflective mirror 100, a first bottom-reflective sheet 102, a secondbottom-reflective sheet 104, a chip holder 90, and the light emittingdiode chip 92. The light emitting diode chip 92 is located on theinclined plane 90A of the chip holder 90. In addition, the shape of thereflective mirror 100 can be elliptic, or any shape that corresponds tothe refractive index of the bottom guide-light board 98.

It is noted that the bottom guide-light board 98 includes an inclinedplane on one side of the bottom guide-light board 98. When the lightemitting diode chip 92 of the chip holder 90 is embedded in the bottomguide-light board 98, the inclined plane 90A of the chip holder 90 canjoint with the inclined plane of the bottom guide-light board 98.

Moreover, a first reflective sheet 102 is located between the topguide-light board 94 and the bottom guide-light board 98. A secondreflective sheet 104 is located under the bottom guide-light board 98.Both the first reflective sheet 102 and second reflective sheet 104 areused to introduce the light source 106 which is emitted from the lightemitting diode chip 92 to become the total reflection light source amongthe bottom guide-light board 98. Thus, the total reflection light sourcecan become the hybrid illumination via the total reflection effect whenthe total reflection light source is reflected among the bottomguide-light board 98. The reflective mirror 100 is positioned on twosides of both the top guide-light board 94 and the bottom guide-lightboard 98.

When the light emitting diode chip 92 emits the light source 106, thelight source would be became the total reflection light source throughthe reflection between the first reflective sheet 102 and the secondreflective sheet 104. The total reflection light source is reflected tothe top guide-light board 94 through the reflective mirror 100. Then,the total reflection light source would be destroyed by the plurality ofmarks 96, such that the destroyed total reflection light source would beemitted to be a light source for the backlight module.

FIG. 8 illustrates the embodiment of the backlight module utilizes theRGB LED chip or with LED chip as the light source according to thepresent invention. It is noted that the several structure as abovediscussion, thus, the same structures or the relationship would not bediscussed. The backlight module includes a top guide-light board 110,and the top guide-light board that includes a plurality of marks 114therein and a reflective sheet 110A thereon. The backlight module alsoincludes at least two bottom guide-light boards 112, the plurality offirst reflective sheets 118 and the plurality of second reflective sheet120, the plurality of reflective mirrors 117, two chip holders 122, andtwo light emitting diode chips 124. Each two light emitting diode chips124 is located on each inclined plane of each two chip holders 122separately, and each light emitting diode chips 124 is embedded in theinclined plane of the bottom guide-light board 112 which is locatedunder the top guide-light board 110.

The light emitting diode chip 124 can emit the light source 126 at aspecific included angle from the inclined plane of the chip holder 122to provide the light source 126 with good illumination efficiency andhigh brightness. The range of the specific included angle is larger than90 degree and is smaller than 180 degree. The first reflective sheet 118is located between the top guide-light board 110 and the bottomguide-light board 112, and the second reflective sheet 120 is locatedunder the bottom guide-light board 112. Both first reflective sheet 118and second reflective sheet 120 are used to introduce the light source126 to cause the total reflection light source among the bottomguide-light board 112. Thus, the total reflection light source would behybridized to be the hybrid illumination.

When the light emitting diode chip 124 emits the light source 126, thelight source 126 would become the total reflection light source throughthe reflection effect between the first reflective sheet 118 and thesecond reflective sheet 120. The total reflection light source isreflected to the top guide-light board 110 through the reflective mirror116. Then, the total reflection light source would be destroyed by theplurality of marks 114, such that the destroyed total reflection lightsource would be emitted out from the top guide-light board 110 to be alight source for the backlight module.

FIG. 9A illustrates the embodiment of the backlight module with thelight emitting diode chip as the light source according to the presentinvention. FIG. 9B shows the completely structure for the backlightmodule with the light emitting diode chip according to the embodiment ofFIG. 9A of the present invention. Referring to FIG. 9A, the backlightmodule includes a top rigid guide-light board 130, a bottom guide-lightboard 132 with a cave 134 therein, the plurality of first reflectivesheets 144, the plurality of second reflective sheets 146, the pluralityof chip holders 138, and the plurality of light emitting diode chips140, wherein the each the plurality of light emitting diode chips 140 islocated on each inclined plane 138A of the plurality of chip holders138. The side of the cave 134 of the bottom guide-light board 132 is aninclined plane, which corresponding to the inclined plane 138A of thechip holder 138. When the bottom guide-light board 132 is combined withthe top guide-light board 130, the cave 134 of the bottom guide-lightboard 132 can match with the inclined plane 138A of the chip holder 138.

Also referring to FIG. 9A, the chip holder 138 with the light emittingdiode chips 140 are located on four sides of the inclined plane 138A ofthe chip holder 138. The chip holder 138 is assembled the bottomguide-light board 132. Thus, the light emitting diode chips 140 canembed in the inclined plane of the bottom guide-light board 132 as shownin FIG. 9B. If the light emitting diode chips 140 is RGB light emittingdiode chip, the reflective sheet (not shown) would be utilized in thebacklight module to introduce the light source to become the totalreflection light source, so as to cause the hybrid illumination amongthe bottom guide-light board 132. Then, the hybrid illumination would bereflected through the reflective mirror (not shown) to the topguide-light board 130. Later, the optical path of the hybridillumination is destroyed by way of the plurality of marks 136, and thenthe hybrid illumination would be emitted out from the top guide-lightboard 130 to be a light source for the backlight module.

What are described above are only the preferred embodiments of thepresent invention, which are not used to limit the claims of the presentinvention; as for the above description, professionals that are familiarwith the present technical field are able to understand and put intopractice, and therefore, the equivalent changes or modifications madewithin the spirit disclosed by the present invention should be includedin the appended claims.

1. A light emitting diode chip backlight module, comprising: a top guide-light board; a bottom guide-light board located under said top guide-light board; a light emitting diode chip; a chip holder having a flat plane toward a side of said bottom guide-light board, and an included angle between said flat plane and a inclined plane of said bottom guide-light board, said light emitting diode chip located on said inclined plane; and a reflective mirror located on said bottom guide-light board and one side of said top guide-light board.
 2. The light emitting diode chip backlight module according to claim 1, wherein said top guide-light board comprises a plurality of marks therein.
 3. The light emitting diode chip backlight module according to claim 1, wherein said chip holder comprises a metal substrate.
 4. The light emitting diode chip backlight module according to claim 1, wherein said light emitting diode chip comprises a red, green, and blue (RGB) light emitting diode chip.
 5. The light emitting diode chip backlight module according to claim 1, wherein said light emitting diode chip comprises a white light emitting diode chip.
 6. The light emitting diode chip backlight module according to claim 1, further comprising a first reflective sheet, a second reflective sheet, and a third reflective sheet, said first reflective sheet located between said top guide-light board and said bottom guide-light board, said second reflective sheet located between said first reflective sheet and said bottom guide-light board, and said third reflective sheet located under said bottom guide-light board.
 7. The light emitting diode chip backlight module according to claim 1, wherein said included angle is of about large than 90 degrees and is less than 180 degrees.
 8. The light emitting diode chip backlight module according to claim 1, further comprising an epoxy resin covered said light emitting diode chip.
 9. The light emitting diode chip backlight module according to claim 1, wherein said light emitting diode chip is embedded in said bottom guide-light board.
 10. A light emitting diode chip backlight module, comprising: a top guide-light board; a bottom guide-light board having an cave therein, and said bottom guide-light board located under said top guide-light board; a light emitting diode chip; a chip holder located on a side of said cave of said bottom guide-light board, said chip holder having a inclined plane toward said side of said cave of said bottom guide-light board and an included angle between said inclined plane and said flat plane of said bottom guide-light board, and said light emitting diode chip is positioned on said inclined plane of said chip holder; and a reflective mirror located on an outside of said bottom guide-light board and a side of said top guide-light board.
 11. The light emitting diode chip backlight module according to claim 10, wherein said top guide-light board comprises a plurality of marks therein.
 12. The light emitting diode chip backlight module according to claim 10, wherein said chip holder comprises a metal substrate.
 13. The light emitting diode chip backlight module according to claim 10, wherein said light emitting diode chip is embedded into said bottom guide-light board.
 14. The light emitting diode chip backlight module according to claim 10, further comprising a first reflective sheet, a second reflective sheet, and a third reflective sheet, said first reflective sheet located between said top guide-light board and said bottom guide-light board, said second reflective sheet located between said first reflective sheet and said bottom guide-light board, and said third reflective sheet located under said bottom guide-light board.
 15. The light emitting diode chip backlight module according to claim 10, wherein said light emitting diode chip comprises a red, green, and blue (RGB) light emitting diode chip.
 16. The light emitting diode chip backlight module according to claim 10, wherein said light emitting diode chip comprises a white light emitting diode chip.
 17. The light emitting diode chip backlight module according to claim 10, wherein said included angle is of about large than 90 degrees and is less than 180 degrees. 